In Chapter 18 of Catcher in the Rye, there are several inferences that the reader can make. Let's pick two of the most intriguing:

"Every time you mention some guy that's strictly a bastard--very mean, or very conceited and all--and when you mention it to the girl, she'll tell you he has an inferiority complex" (73). This takes place in the beginning of Chapter 18 when the narrator is discussing one of Jane's former dates, Al Pike. The narrator is confused by the fact that any boy who he thinks is of poor quality and character - which can range from thinking he is "very hot stuff" to being "all muscles and no brains" - is defended by girls who feel bad for him and claim that he is afraid of looking like he is worse than the people around him (73). There is an important inference about Holden that the critical reader can identify: he is being deprecating towards other boys, calling them bastards, mean, and conceited, but this is a characteristic of somebody with an inferiority complex! A common symptom of inferiority complexes is aggressive behavior towards others to compensate for one's own feelings of inadequacy, much like Holden himself is engaging in.

" I told him I liked Ring Lardner and The Great Gatsby and all. I did, too. I was crazy about The Great Gatsby" (76). The inference here requires a little bit of background knowledge about Fitzgerald's novel The Great Gatsby. In it, the main character is a man who completely reinvents himself, changing his name, making a fortune, creating a new history, and completely transforming his lifestyle, all to impress a woman. By definition, everything about him is "phony" because it is fictional. However, Holden claims that he loved not only the novel but also the main character, "Old Gatsby. Old Sport" (76). The inference here is that Holden's judgement of a person as "phony" is pretty much entirely subjective, and also depends on whom he is with.

TL;DR - Chapter 18 infers that Holden has an inferiority complex and that his description of people as phony is subjective.